This invention relates generally to satellite-inertial navigation systems and more particularly to methods and apparatus for determining the attitude of platforms in a satellite-inertial navigation environment.
The Global Positioning System (GPS), the current exemplar of a satellite navigation system, consists of 24 globally-dispersed satellites with synchronized atomic clocks. Each satellite transmits a coded signal having the satellite clock time embedded in the signal and carrying information concerning the emphemerides of the satellites and its own daily emphemeris and clock corrections. A user obtains the essential data for determining his position and clock error by measuring the differences in his receiver clock time and the satellite clock times embedded in the signals from at least four viewable satellites. The difference in receiver clock time and satellite clock time multiplied by the radio wave propagation velocity is called the pseudorange and is equal to the range to the satellite plus the incremental range equivalent of satellite clock error minus the receiver clock error.
The user also obtains the essential data for determining his velocity by measuring for each satellite the difference in the frequency of the actual satellite signal and the frequency of the satellite signal if it had been generated using the receiver clock. The accumulated change in phase over a fixed period of time resulting from this frequency difference expressed in units of distance is called the delta range and is equal to the change in satellite range over the fixed period of time plus the change in the difference in the receiver and satellite clocks over the same fixed period of time multiplied by the radio wave propagation velocity.
The user, knowing the positions, velocities, and clock errors of the satellites, can compute his own position, velocity, and clock error from the measured pseudoranges and delta ranges.
Since the more significant errors in GPS-determined positions of nearby platforms are highly correlated, these errors tend to cancel out in determining the relative positions of the platforms. The use of GPS for making highly-accurate relative position determinations of nearby platforms is referred to as differential GPS.
The accuracy attainable with differential GPS suggests the use of interferometric GPS for determining the attitude of a platform. Interferometric GPS denotes the use of satellite signal carrier phase measurements at different points on a platform for accurately determining the orientation of the platform (and the other Kalman states) from highly-accurate position updates.
The use of three spatially-distributed antennas on a platform permits the accurate determination with GPS signals alone of pitch, roll, and heading. However, if the platform is a highly-maneuverable aircraft, it becomes necessary to integrate the platform GPS equipment with an inertial navigation unit. GPS compensates for inertial navigation system drifts and when platform maneuvering or other occurrences causes GPS to become temporarily inoperative, the inertial navigation system (INS) carries on until the GPS again becomes operative.